The study of petroleum analysis and its products would not be complete without considering the incompatibility that generates changes in its original properties. During and after the blending process, various secondary products can be formed such as sludge, semi-solids or solid particles increasing mixture viscosity. The term incompatibility refers to the formation of a precipitate (sludge, sediment and deposition of material with asphaltene content) or separation of phases when two liquids are blended (Speight 1999, 2004).
The phenomenon of incompatibility was firstly used by Martin (1951) defining it as the tendency of the fuel oil to produce a deposit in the dilution or blending with other fuel oils. Martin (1951) defined a difference between incompatibility and instability, as the tendency of a residual fuel to produce a deposit of asphaltic or carbonic material during storage or heating process. Instability during storage or during heating can be the result of the preparation of the fuel from incompatible components or can be the result of aging. The term instability is frequently used referring to color formation, sediment or bubble gum in the liquid during a period of time; this term can be used to differentiate the formation of a precipitate in short time (almost immediately). Nevertheless, the terms incompatibility and instability are used interchangeably (Speight, 1999).
The phenomena of incompatibility and instability of petroleum and its products are invariably associated with the chemical composition and physical ratio of the components. In the most of cases, a certain component in one the crudes reacts with another component in the crude with which is blended resulting in a chemical reaction in the formation of a new product that, when it is soluble, affects the mixture properties and when it is insoluble, it is deposited as a semisolid or solid matter (Speight, 1999). Normally, the incompatibility processes increase viscosity of petroleum and its derivatives, inclusively at low temperatures also causes a change in viscosity in certain fuels (Speight, 1999). Various studies demonstrated that blending of different crudes can lead to flocculation/precipitation of asphaltenes (Wiehe and Kennedy, 2000a, 2000b; van den Berg et al., 2003; Schermer et al., 2004). This phenomenon, known as crude incompatibility, causes problems in the transportation and refining process especially when the economical situation is requiring many refineries to carry out low cost crude blending to improve the refining margins (van den Berg et al., 2003).
Instead of various studies carried out in the last decades, there are always important questions in the chemistry and physics knowledge of the incompatibility phenomenon (Speight, 1999; Wiehe, 2012). It is well know that this problem did not lead to a standard method for determination and quantification of crude incompatibility. Based on the above, there are various criteria for determining crude incompatibility in literature.
U.S. Pat. No. 4,853,337 refers to a blending procedure of liquid hydrocarbons to control incompatibility mentioning that the paraffinic and condensed liquids can be blended with the crudes while the incompatibility of the asphaltenes is controlled; the incompatibility is expressed as the relation between the aromatics and the asphaltenes content of the crudes or liquid hydrocarbon.
Escobedo and Mansoori (1995) determine an incipient point in asphaltenes flocculation by means of relative viscosity measurement of a crude diluting it with a precipitating agent (n-pentane, n-heptane, n-nonane). The phenomenon is graphically shown with the increase of viscosity during the precipitation of the crude observing a deviation of the initial behavior during asphaltenes flocculation.
Asomaning (1997) shows the incompatibility phenomenon associating its solubility as the mechanism for deposits formation. Later, Asomaning and Watkinson (2000) introduced an index of simple colloidal instability (CII) based on the analysis of saturated, aromatic compositions, resins and asphaltenes (SARA) of the crudes mixture, concluding that mixtures with CII>1 tend to be incompatible and precipitate asphaltenes.
Buckley et al. (1998) associates the incipient point of precipitation to asphaltenes with solubility that depends on the refraction index (RI); in this work it is determined that the RI could be used to predict the incipient point of asphaltene precipitation. See also Buckley (1999) and Giménez and Cabeza (2006).
U.S. Pat. No. 5,871,634 refers to a method for blending two or more petroleum feedstreams, petroleum process streams or combination thereof, at least one of which includes the steps of determining the insolubility number, (IN) for each feedstream, determining the solubility blending number, SBN, for each feedstream and combining the feedstreams in order of decreasing SBN number of each feedstream such that the solubility blending number of the mixture is greater than the insolubility number of any component of the mix, when the solubility blending number of any of the feedstreams or streams is equal or less than the insolubility number of any of the streams. See also the works of Wiehe and Kennedy (2000a, 2000b).
U.S. Pat. No. 5,997,723 refers to a process for blending crudes with the purpose of avoiding incrustation for crudes considered almost incompatible. See also the works of Wiehe et al. (2001) and Wiehe (2004).
Gharfeh et al. (2004) disclose an instrument for detection of diluted crudes incompatibility with heptane at low temperatures and atmospheric pressure. The measurement system consists in a titration container, an infrared laser and a detector for measuring light transmittance through the container. Initially, the transmittance increases and when it arrives to a flocculation point, it starts to decrease, then, the maximum point achieved is considered as the flocculation point of asphaltenes or the maximum dilution achieved.
U.S. Pat. No. 7,029,570 refers to a process for determining incompatibility in the crudes mixtures through the change in length density of neutrons dispersion at the surface of asphaltene aggregates.
U.S. Pat. No. 7,618,822 refers to crudes processing, mixtures and fraction in refines and petrochemical plants to decrease asphaltenes flocculation in the pre-heating train interchanger, ovens and other units of the refining process.
Flakier and Sandu (2010) disclose a technique for providing information on crudes stability on its mixtures detecting the incipient point of the asphaltenes flocculation based on very small changes of the composition of the mixtures using a source of transmission close to the infrared. The equipment used in this way has a detection system for solids able to measure the changes of intensity through the addition of a precipitate (n-pentane). An inflexion point of a transmittance graph based on the volume of precipitate added up to the start of flocculation can be observed. The inflexion point is expressed as the stability index of asphaltenes that corresponds to the precipitation point of asphaltenes and provides a relative measurement of the stability in the crude. Meanwhile, Sedghy and Goual (2010) determine the start of asphaltenes precipitation through the direct current conductivity technique in crudes diluted in toluene. Alvarez et al. (2012) use an ellipsometry technique to evaluate the compatibility of crudes mixtures.
Mexican Patent application No. MX/2011/003287, published Sep. 27, 2012 refers to a process for measuring dynamic viscosity of live heavy crude (monophasic samples taken from the well bottom) at a constant temperature and pressure from 68.9 MPa up to atmospheric pressure, including the dynamic viscosity in the bubble pressure point and under this least; that is, removing the gaseous phase and measuring liquid phase viscosity up to achieving atmospheric pressure.
Thus, an object of the present invention is to blend heavy and light crudes to decrease viscosity of the heavy crudes and to find the optimum concentration for maintaining asphaltenes below a predetermined level to reduce the tendency for asphaltene formation. According to the present invention, an incipient point of the asphaltene incompatibility threshold in the mixture is obtained that is a very important point to avoid because of the formation of asphaltene aggregates at a certain concentration of light crudes; the blending process comprises crudes with asphaltenes content.
The term “dead crude” as used in the present application is defined as a crude that has a sufficiently low pressure, does not contain dissolved gas or did not release its volatile components.
Similarly, the API gravity is a measure of density that describes how heavy or light the petroleum is compared with water. If the API gravity of the petroleum is more than 10, it is lighter than the water and therefore will float on it. The API gravity is also used to compare densities of fractions extracted from petroleum. For example, if a petroleum fraction floats on another; it means that it is lighter and therefore its API gravity is higher. Mathematically, the API gravity does not have units; nevertheless, this number always has the name API grade. The API gravity is measured with an instrument denominated densitometer; there are a great variety of these instruments. The °API of crudes oils, generally are in an interval of 47° (for lighter crudes) to 10° (for heavier crudes). Based on this parameter it is possible to classify the crudes in: extra heavy (°API<10), heavy (10.1<°API<22.3), mean or middle (22.4<°API<31.1), light (31.2<°API<38.9) and extra light (39.0<°API). It is important to mention that this classification can vary depending on the considered source.
The “room temperature” is the temperature where the laboratory is installed without having an external control of same, wherein any measurement is carried out, i.e. depending of the place where it can be found. For purposes of the description of the present invention, “room temperature” will be considered between 293.2 and 298.2 K.
Currently, there is no any reliable technique to determine the crude mixture incompatibility. Therefore, the present invention markedly overcomes prior techniques, since it provides the ratios of blending wherein the incompatibility of light and heavy crudes can occur through the determination of dynamic viscosity.
Therefore, another object of the present invention is to provide a process for measuring the dynamic viscosity of heavy and light crude mixtures with an apparatus containing a sensor based on a technique involving constant electromagnetic force.
Another object of the current invention is to provide a process for determining crude mixture incompatibility by measuring the dynamic viscosity of the mixtures at different temperature and pressure conditions.